We propose to study the role of DNA-dependent RNA polymerase in gene regulation during the differentiation of bacteria into spores. We have suggested that changes in gene expression during sporulation may be in part regulated by changes in the template specificity and subunit composition of RNA polymerase. The onset of sporulation by Bacillus subtilis is associated with a marked decrease in the ability of the sigma subunit of RNA polymerase to direct active transcription of phage phi e DNA in vitro. Although sporulating cells contain as much sigma polypeptide as vegetative bacteria, sigma from sporulating B. subtilis is functionally inhibited and only weakly associated with RNA polymerase. The inhibition of sigma is accompanied by the appearance of at least two new polypeptides of 70,000 and 90,000 daltons that bind specifically to RNA polymerase. Our goal is to determine the mechanism by which sigma is dissociated from RNA polymerase during spore formation and to determine whether the 70,000 or 90,000 dalton RNA polymerase-binding proteins direct the transcription of sporulation messenger RNAs in vitro from B. subtilis DNA as a template.